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Three drug resistant mutants of Dictyostelium discoideum and D. mucoroides were employed to demonstrate the occurrence of interspecific hybridization. Interspecific hybrids appeared at a frequency of 4 x 10-5, whereas intraspecific hybrids appeared at a frequency of 1 x 10-3. Nineteen interspecific hybrids were serially subcultured to investigate their genetic traits. The results showed that the progenies of the interspecific hybrids retained the genetic markers of both parents in a manner similar to intraspecific hybrids. The characterization of the interspecific hybrids demonstrated that parasexual hybridization occurred even between two different species. The present study showed that genetic analysis between different species of cellular slime molds could be performed by applying this interspecific parasexual system. A novel sexual process has been established in the cellular slime mold Dictyostelium discoideum, and various genetic markers have been revealed to recombine during this process (10, 12, 15). Recently, linkage analysis was performed and mitotic crossing-over was confirmed (11, 13). This exchange process of genetic materials in this organism differs from that of ordinary sexual reproduction, and a useful term to describe this process is "parasexual," i.e., "a process of genetic recombination other than a regular alteration of karyogamy and meiosis as in sexual reproduction" (17). Investigations of the parasexual cycle so far in this organism have revealed the following points : (a) the frequency of appearance of heterozygous cells was 10-4 to 10-6 in standard selective conditions (10, 14, 23); (b) Mendelian segregation resulting from meiosis was not evident while chromosome elimination was suggested to occur (1, 10); (c) the diploid state of heterozygous cells was unstable and rapid haploidization occurred during the course of progeny multiplication (20, 22); and (d) all mutants of parasexual genetics employed so far were derived from the common strain NC-4, indicating that no mating types are involved in this process. The occurrence of true sexuality involving meiosis was demonstrated during the process of macrocyst formation in Polysphondylium violaceum and D. mucoroides (3, 16). This process involves mating type systems, and a strain not forming macrocysts is heterothallic strain (2, 4). The frequency of gene recombinations occurring through *Present address : The Research Institute for Microbial Diseases , Osaka University, Suita, Osaka 565, Japan.

cell structure and function

Parasexual Hybridization in Cellular Slime Molds.II. The Appearance and Characterization of Interspecific Hybrids between Dictyostelium discoideum and D. mucoroides